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The properties of polyaniline-polyelectrolyte complexes
ELSEVIER
Synthetic
The Properties
Metals
84 (1997)
77-78
of Polyaniline-Polyelectrolyte
Complexes
S. M. Yang, W. M. Chen and K. S. You Department of Chemistry, National Central University Chung-Li, Taiwan Abstract Water-soluble polyaniline-polyelectrolyte and the degrees of electron delocalization methods. Keywords:
Electron
spin resonance,
complexes of polyaniline
photoelectron
The protonation levels, oxidation states are synthesized. in the complexes are analyzed via various spectroscopic
spectroscopy,
Introduction Polyaniline is a conducting polymer with interesting properties. The stability of polyaniline in air provides the opportunity of various applications. However, the solubilities of polyaniline salts with small counter anions are limited in common organic solvents. Heeger and coworkers1 use surfactant anions successfully to improve the solubility of polyaniline in m-cresol and o-xylene. Liu and coworkers’ reported the synthesis of water soluble polyaniline polyelectrolyte complexes. In this study we report the characterization of water soluble polyaniline polyelectrolyte complexes with various spectroscopic methods to study the protonation level, oxidation state and degree of electron delocalization of polyaniline. Experimental Freshly distilled aniline was added into an aqueous solution of polyacid (polyacrylic acid, polystyrenesulfonic acid). After stirring, an aqueous solution containing hydrochloric acid and oxidant (ammonium peroxydisulfate or hydrogen peroxide) was added with vigorous stirring. Several hours later, the solution was dialyzed in an acidic medium to remove unreacted aniline monomer. The products were dried and characterized with a Digilab FT-40 FTIR, Milton Roy spectronic 3000 UV-VIS spectrometer, Bruker ER 200 DSRC EPR spectrometer, and VG Escalab MKII XPS spectrometer. Results and discussion IR spectra Additional absorption peaks appear at 2920, 2600, 1500, 740, 690, and 480 cm-’ after addition of aniline to polyacrylic acid. Those are the characteristic absorption peaks of anilinium salts. Addition of oxidants lead to the appearance of absorption peaks at 1570, 1490, 1305, 1120 0379-5779/97/$17.00
PII
SO379A779(96)03844-1
Q 1997
Elsevier
Science
S.A
All
rights
r-ed
optical
absorption
spectroscopy
and 800 cm-l indicating the formation of emeraldine salt. There are still some peaks of anilinium salts. After dialysis the peaks indicating anilinium salts disappear. IR spectra of polyaniline polystyrenesulfonic acid complexes (PANI-PSSA) also show the characteristic absorption peaks of emeraldine salt. The complexes formed with ammonium pero
spectra
The spectra of polyaniline polyacrylic acid complex (PANI-PAA) and polyaniline polystyrenesulfonic acid complex (PANI-PSSA) in aqueous media of pH 8 still show absorption peaks at 800-900 nm and a shoulder at 420 nm which are the absorption peaks of emeraldine salt. Emeraldine salt of small counter ions are usually dedoped in media of pH 4. An absorption peak at 550-630 nm appears and the peaks at 420 nm and 800 nm disappear after adjusting the pH of the medium to 11. The spectra change indicates the formation of base form. The absorption peak of PANI-PAA complex after dedoping appears at 620 nm which indicate the emeraldine oxidation state. However the absorption peak of PANI-PSSA synthesized with ammonium peroxydisulfate and hydrogen peroxide shifts to 553 nm and 573 nm respectively indicating the higher oxidation state then emeraldine. Addition of reductant (hydrazine) to PANI-PAA and PANI-PSSA, only absorption peaks at 320-330 nm left. The spectra indicate the formation of reduced leucoemeraldine state. Addition of oxidant (ammonium peroxydisulfate), absorption peak at 620 nm of PANI-PAA shifts to 548 nm,
78
SM.
Yang et 01. /Synthetic
indicating pernigraniline state. PANI-PSSA is not stable on addition of oxidant, the absorption peak at 560 nm shifts to 780 nm and then shift back to 660 nm then disappear. Only weak absorption at 380 nm left. X-ray
photospectra
Nr, peak of PANI-PAA can be deconvoluted into three peaks with binding energy of 398.0 eV, 399.2 eV and larger than 400 eV. The peaks correspond to imine nitrogen (398.0.eV) in oxidized units, amine nitrogen (399.2 eV) in reduced units and protonated nitrogen (larger than 400 eV). The peak area of protonated nitrogen is 22% of total nitrogen which indicate the protonation level of polyaniline is 22%. After dedoping, only imine and amine nitrogens appear. The peak area of amine nitrogen is 49% of total nitrogen. The results agree with the emeraldine oxidation state of the complex. Cl, peak can be deconvoluted into four kinds of chemical environments: CH with binding energy 284.6 eV, C-O with binding energy 286.0 eV, C=O with binding energy of 287.5 eV and COOH with binding energy 288.9 eV. Chemical environments of C-O and C=O arise from surface oxide. The ratio of the peak area COOH/N = 1.8, indicating the ratio of acrylic acid monomer to aniline monomer is 1.8. Twenty-two percent of the aniline units are protonated. We can estimate the free acid groups in PANI-PAA is 87.8% of the total acid groups. In the other words, one of eight acid groups protonate the nitrogen of polyaniline. If we assume PANI-PAA complex consists of one polyaniline chain surrounded by one polyacrylic acid (molecular weight 90000) chain, we can estimate the molecular weight of polyaniline is 63000 in the complex. Kang and coworkers3 use a similar method to synthesize PANIPAA. They also obtain the protonation level of 25% for polyanihne. The polyaniline doped with polyacrylic acid of molecular weight 70000 shows COOH/N ratio of 6.6-13.0. In this study, PANI-PAA with COOH/N = 1.8 which is much lower than polyacrylic acid doped polyaniline (COOH/N = 6.6-13). Less free acid groups on PANI-PAA indicate the formation of complex. Low protonation level (22%) of polyaniline is attributable to the weak acidity of carboxyhc groups. Steric effect may also contribute to the low protonation level. Nr, peak of PANI-PSSA can be deconvoluted into two peaks with chemical environments corresponding to N in amine unit and protonated form. The area of protonated N is 84% of total nitrogen which means extra high protonation level. Strong acid group -SOsH may protonate not ,only the imine nitrogen but also the amine nitrogen. High oxidation state of PANI-PSSA as indicated from IR and UV-VIS spectra consisting of more imine nitrogen may also be responsible for the high protonation level. From the area of Sap and N1, peaks, the ratio of S/N = 1.3 can be obtained. There are 84% of total nitrogen being protonated. The free acid groups in PANI-PSSA is 35.4% of total acid groups which means two of three acid groups protonate ni-
Metals
84 (1997)
77-78
trogen. If we also assume a model of one polyaniline chain surrounded by a polystyrenesulfonic acid chain (molecular weight 70000), one can estimate the molecular weight of polyaniline is around 26000. It is possible that there are some HCl doping of the polyaniline as well as the polyacid since the oxidant is added from a hydrochloric acid solution. The low doping level of PANI-PAA seems to indicate not much HCl doping involved in this complex. Further works on the syntheses of PAN1 complexes without adding HCl solution are under investigation. EPR Spectra Epr spectra all show a single Lorenzian shape signal without hyperfine structure. The g values of these complexes are close to that of emeraldine hydrochloric acid salt. The peak to peak linewidths (A H,,) of PANI-PAA complexes are much larger than that of emeraldine hydrochloric acid salt. While the peak to peak linewidths of PANI-PSSA complexes are close to that of emeraldine hydrochloric acid salt. The low protonation levels (22%) of PANI-PAA complexes lower the degree of delocalization and lead to the wide signal. On the other hand, the high protonation level (84%) of PANI-PSSA results in sharp epr signal. The peak to peak linewidths of the complexes synthesized with hydrogen peroxide as the oxidant are wider than the complexes synthesized with ammonium peroxydisulfate as the oxidant. That is probably attributable to the higher molecular weight of the complexes synthesized with ammonium peroxydisulfate. Conclusion Water-soluble polyaniline-polyelectrolyte complexes are synthesized via template-guided scheme. The protonation levels of PANI-PAA and PANI-PSSA complexes are 22% and 84%, respectively. The oxidation state of PANI-PAA is close to while the oxidation state of PANI-PSSA is higher than those of emeraldine salts. The degrees of electron delocalization of PANI-PSSA are close to the emeraldine hydrochloric acid salts. The degree of electron delocalization is lower in PANI-PAA, that is probably attributable to the lower protonation level of polyaniline in PANI-PSSA. Acknowledgement Financial support from NSC, Republic of China, under the grant NSC84-2113-M008-002 is acknowledged. The authors thank professor E.rT. Kang of National University of Singapore for XPS spectra measurements. References 1. Y. Cao, P. Smith and A. J. Heeger, Synth. Met., 1992, 48, 91. 2. J. M. Liu and S. C. Yang, J. Chem. Sot., Chem. Commun., 1991, 1529. 3. E. T. Kang, K. G. Neoh and K. L. Tan, Polymer, 1994, 35(15), 3193.
Synthetic
The Properties
Metals
84 (1997)
77-78
of Polyaniline-Polyelectrolyte
Complexes
S. M. Yang, W. M. Chen and K. S. You Department of Chemistry, National Central University Chung-Li, Taiwan Abstract Water-soluble polyaniline-polyelectrolyte and the degrees of electron delocalization methods. Keywords:
Electron
spin resonance,
complexes of polyaniline
photoelectron
The protonation levels, oxidation states are synthesized. in the complexes are analyzed via various spectroscopic
spectroscopy,
Introduction Polyaniline is a conducting polymer with interesting properties. The stability of polyaniline in air provides the opportunity of various applications. However, the solubilities of polyaniline salts with small counter anions are limited in common organic solvents. Heeger and coworkers1 use surfactant anions successfully to improve the solubility of polyaniline in m-cresol and o-xylene. Liu and coworkers’ reported the synthesis of water soluble polyaniline polyelectrolyte complexes. In this study we report the characterization of water soluble polyaniline polyelectrolyte complexes with various spectroscopic methods to study the protonation level, oxidation state and degree of electron delocalization of polyaniline. Experimental Freshly distilled aniline was added into an aqueous solution of polyacid (polyacrylic acid, polystyrenesulfonic acid). After stirring, an aqueous solution containing hydrochloric acid and oxidant (ammonium peroxydisulfate or hydrogen peroxide) was added with vigorous stirring. Several hours later, the solution was dialyzed in an acidic medium to remove unreacted aniline monomer. The products were dried and characterized with a Digilab FT-40 FTIR, Milton Roy spectronic 3000 UV-VIS spectrometer, Bruker ER 200 DSRC EPR spectrometer, and VG Escalab MKII XPS spectrometer. Results and discussion IR spectra Additional absorption peaks appear at 2920, 2600, 1500, 740, 690, and 480 cm-’ after addition of aniline to polyacrylic acid. Those are the characteristic absorption peaks of anilinium salts. Addition of oxidants lead to the appearance of absorption peaks at 1570, 1490, 1305, 1120 0379-5779/97/$17.00
PII
SO379A779(96)03844-1
Q 1997
Elsevier
Science
S.A
All
rights
r-ed
optical
absorption
spectroscopy
and 800 cm-l indicating the formation of emeraldine salt. There are still some peaks of anilinium salts. After dialysis the peaks indicating anilinium salts disappear. IR spectra of polyaniline polystyrenesulfonic acid complexes (PANI-PSSA) also show the characteristic absorption peaks of emeraldine salt. The complexes formed with ammonium pero
spectra
The spectra of polyaniline polyacrylic acid complex (PANI-PAA) and polyaniline polystyrenesulfonic acid complex (PANI-PSSA) in aqueous media of pH 8 still show absorption peaks at 800-900 nm and a shoulder at 420 nm which are the absorption peaks of emeraldine salt. Emeraldine salt of small counter ions are usually dedoped in media of pH 4. An absorption peak at 550-630 nm appears and the peaks at 420 nm and 800 nm disappear after adjusting the pH of the medium to 11. The spectra change indicates the formation of base form. The absorption peak of PANI-PAA complex after dedoping appears at 620 nm which indicate the emeraldine oxidation state. However the absorption peak of PANI-PSSA synthesized with ammonium peroxydisulfate and hydrogen peroxide shifts to 553 nm and 573 nm respectively indicating the higher oxidation state then emeraldine. Addition of reductant (hydrazine) to PANI-PAA and PANI-PSSA, only absorption peaks at 320-330 nm left. The spectra indicate the formation of reduced leucoemeraldine state. Addition of oxidant (ammonium peroxydisulfate), absorption peak at 620 nm of PANI-PAA shifts to 548 nm,
78
SM.
Yang et 01. /Synthetic
indicating pernigraniline state. PANI-PSSA is not stable on addition of oxidant, the absorption peak at 560 nm shifts to 780 nm and then shift back to 660 nm then disappear. Only weak absorption at 380 nm left. X-ray
photospectra
Nr, peak of PANI-PAA can be deconvoluted into three peaks with binding energy of 398.0 eV, 399.2 eV and larger than 400 eV. The peaks correspond to imine nitrogen (398.0.eV) in oxidized units, amine nitrogen (399.2 eV) in reduced units and protonated nitrogen (larger than 400 eV). The peak area of protonated nitrogen is 22% of total nitrogen which indicate the protonation level of polyaniline is 22%. After dedoping, only imine and amine nitrogens appear. The peak area of amine nitrogen is 49% of total nitrogen. The results agree with the emeraldine oxidation state of the complex. Cl, peak can be deconvoluted into four kinds of chemical environments: CH with binding energy 284.6 eV, C-O with binding energy 286.0 eV, C=O with binding energy of 287.5 eV and COOH with binding energy 288.9 eV. Chemical environments of C-O and C=O arise from surface oxide. The ratio of the peak area COOH/N = 1.8, indicating the ratio of acrylic acid monomer to aniline monomer is 1.8. Twenty-two percent of the aniline units are protonated. We can estimate the free acid groups in PANI-PAA is 87.8% of the total acid groups. In the other words, one of eight acid groups protonate the nitrogen of polyaniline. If we assume PANI-PAA complex consists of one polyaniline chain surrounded by one polyacrylic acid (molecular weight 90000) chain, we can estimate the molecular weight of polyaniline is 63000 in the complex. Kang and coworkers3 use a similar method to synthesize PANIPAA. They also obtain the protonation level of 25% for polyanihne. The polyaniline doped with polyacrylic acid of molecular weight 70000 shows COOH/N ratio of 6.6-13.0. In this study, PANI-PAA with COOH/N = 1.8 which is much lower than polyacrylic acid doped polyaniline (COOH/N = 6.6-13). Less free acid groups on PANI-PAA indicate the formation of complex. Low protonation level (22%) of polyaniline is attributable to the weak acidity of carboxyhc groups. Steric effect may also contribute to the low protonation level. Nr, peak of PANI-PSSA can be deconvoluted into two peaks with chemical environments corresponding to N in amine unit and protonated form. The area of protonated N is 84% of total nitrogen which means extra high protonation level. Strong acid group -SOsH may protonate not ,only the imine nitrogen but also the amine nitrogen. High oxidation state of PANI-PSSA as indicated from IR and UV-VIS spectra consisting of more imine nitrogen may also be responsible for the high protonation level. From the area of Sap and N1, peaks, the ratio of S/N = 1.3 can be obtained. There are 84% of total nitrogen being protonated. The free acid groups in PANI-PSSA is 35.4% of total acid groups which means two of three acid groups protonate ni-
Metals
84 (1997)
77-78
trogen. If we also assume a model of one polyaniline chain surrounded by a polystyrenesulfonic acid chain (molecular weight 70000), one can estimate the molecular weight of polyaniline is around 26000. It is possible that there are some HCl doping of the polyaniline as well as the polyacid since the oxidant is added from a hydrochloric acid solution. The low doping level of PANI-PAA seems to indicate not much HCl doping involved in this complex. Further works on the syntheses of PAN1 complexes without adding HCl solution are under investigation. EPR Spectra Epr spectra all show a single Lorenzian shape signal without hyperfine structure. The g values of these complexes are close to that of emeraldine hydrochloric acid salt. The peak to peak linewidths (A H,,) of PANI-PAA complexes are much larger than that of emeraldine hydrochloric acid salt. While the peak to peak linewidths of PANI-PSSA complexes are close to that of emeraldine hydrochloric acid salt. The low protonation levels (22%) of PANI-PAA complexes lower the degree of delocalization and lead to the wide signal. On the other hand, the high protonation level (84%) of PANI-PSSA results in sharp epr signal. The peak to peak linewidths of the complexes synthesized with hydrogen peroxide as the oxidant are wider than the complexes synthesized with ammonium peroxydisulfate as the oxidant. That is probably attributable to the higher molecular weight of the complexes synthesized with ammonium peroxydisulfate. Conclusion Water-soluble polyaniline-polyelectrolyte complexes are synthesized via template-guided scheme. The protonation levels of PANI-PAA and PANI-PSSA complexes are 22% and 84%, respectively. The oxidation state of PANI-PAA is close to while the oxidation state of PANI-PSSA is higher than those of emeraldine salts. The degrees of electron delocalization of PANI-PSSA are close to the emeraldine hydrochloric acid salts. The degree of electron delocalization is lower in PANI-PAA, that is probably attributable to the lower protonation level of polyaniline in PANI-PSSA. Acknowledgement Financial support from NSC, Republic of China, under the grant NSC84-2113-M008-002 is acknowledged. The authors thank professor E.rT. Kang of National University of Singapore for XPS spectra measurements. References 1. Y. Cao, P. Smith and A. J. Heeger, Synth. Met., 1992, 48, 91. 2. J. M. Liu and S. C. Yang, J. Chem. Sot., Chem. Commun., 1991, 1529. 3. E. T. Kang, K. G. Neoh and K. L. Tan, Polymer, 1994, 35(15), 3193.